Reducing Agent Metering System with Evacuation of the Reducing Agent Line after Completion of Metering

ABSTRACT

The invention relates to a reducing agent metering system for injection of a reducing agent into the exhaust gas flow of an internal combustion engine for selective catalytic reduction, the metering system being connected/connectable to a reducing agent tank from which the reducing agent is taken and delivered by means of a delivery pump via a delivery line ( 1 ) and is introduced into the exhaust gas flow of the internal combustion engine via at least one nozzle, a Venturi tube ( 2 ) being integrated into the delivery line ( 1 ) and having a branch ( 3 ) which discharges into the nozzle, at the outlet ( 4 ) of the Venturi tube ( 2 ) there being an openable shutoff element so that with the shutoff element opened the reducing agent which is located in the nozzle and the branch ( 3 ) is intaken as a suction medium into the Venturi tube ( 2 ) and is carried away via the outlet ( 4 ).

The invention relates to a reducing agent metering system for injection of a reducing agent into the exhaust gas flow of an internal combustion engine for selective catalytic reduction, the metering system being connected/connectable to a reducing agent tank from which the reducing agent is taken and delivered by means of a delivery pump via a delivery line and is introduced into the exhaust gas flow of the internal combustion engine via at least one nozzle.

Furthermore the invention relates to a method for operating a reducing agent metering system for injection of a reducing agent into the exhaust gas flow of an internal combustion engine for selective catalytic reduction, the metering system being connected/connectable to a reducing agent tank from which the reducing agent is taken and delivered by means of a delivery pump via a delivery line and is introduced into the exhaust gas flow of the internal combustion engine via at least one nozzle.

Catalytic converters for selective catalytic reduction, so-called SCR-catalytic converters (SCR: selective catalytic reduction) are used to reduce the nitrogen oxide emission from diesel engines, furnace installations, garbage incineration systems, industrial plants and the like. To do this a reducing agent is injected into the exhaust gas system with a metering device. The reducing agent is ammonia or an ammonia solution or another reducing agent.

Since carrying ammonia in motor vehicles is critical with regard to safety, urea in an aqueous solution with conventionally a 32.5% urea proportion according to DIN 70070, so-called AdBlue, is used. In the exhaust gas the urea decomposes at temperatures above 150° Celsius into gaseous ammonia and CO₂. The parameters for the decomposition of urea are essentially time (vaporization and reaction time), temperature and droplet size of the injected urea solution. In these SCR catalytic converters the emission of nitrogen oxides is reduced by roughly 90% by selective catalytic reduction (SCR).

Because the 32.5% solution which is conventionally used freezes at roughly −11° C. and expands by roughly 10%, components of the metering system can be damaged as a result of the expansion of the aqueous solution or of the portion of water of the solution. The problem of freezing also relates to any aqueous solution of a reducing agent other than the 32.5% urea solution which is often used as the reducing agent. As a result of the high proportion of water in the aqueous reducing agent solution, freezing damage can occur due to the anomaly of water at low temperatures.

In order to avoid damage of components of the metering system by expansion as a result of freezing of the reducing agent solution, the components which carry the reducing agent must be cleared of the reducing agent solution in metering pauses and especially after completion of the metering.

Another problem is that the reducing agent solution could crystallize. After completion of injection of the reducing agent the nozzle must therefore be cleared of residues of the reducing agent solution to prevent their crystallizing and clogging the outlet opening of the nozzle.

DE 10 2008 013 960 A1 discloses cleaning the components which carry the reducing agent by means of compressed air after completion of metering. Here the disadvantage is that this procedure requires that a certain minimum pressure be maintained in a compressed air system for a certain time interval after completion of metering and the procedure is not fault-tolerant for example in a failure of the electrical supply of the metering system.

Another disadvantage in this approach is that there must be a compressed air line solely for blowing out the delivery line of the reducing agent and the nozzle, as a result of which the systems engineering cost is increased and the remaining amount of the reducing agent is injected unused into the exhaust gas line.

The object of the invention is to develop a reducing agent metering system of the initially named type such that evacuation of the reducing agent delivery line and the nozzle is enabled in the metering pauses and after completion of metering at reduced system cost, so that these components are reliably cleared of residues of the reducing agent solution in order to prevent freezing damage and clogging without a separate compressed air supply for ventilating the reducing agent delivery line and the nozzle having to be provided. Another object of the invention is to devise a method for operating a reducing agent metering system which enables evacuation of the reducing agent delivery line and the nozzle in the metering pauses and after completion of metering at reduced system cost without the reducing agent delivery line and the nozzle having to be blown out by means of compressed air.

This object is achieved as claimed in the invention by a reducing agent metering system as claimed in Claim 1 and by a method for operating a reducing agent metering system as claimed in Claim 10. Advantageous developments of the invention are given in the respective dependent claims.

In the reducing agent metering system for injection of a reducing agent into the exhaust gas flow of an internal combustion engine for selective catalytic reduction, the metering system being connected/connectable to a reducing agent tank from which the reducing agent is taken and delivered by means of a delivery pump via a delivery line and is introduced into the exhaust gas flow of the internal combustion engine via at least one nozzle, it is especially advantageous that a Venturi tube is integrated into the delivery line and has a branch which discharges into the nozzle, at the outlet of the Venturi tube there being an openable shutoff element so that with the shutoff element opened the reducing agent which is located in the nozzle and the branch is intaken as a suction medium into the Venturi tube and is carried away via the outlet.

In the method for operating a reducing agent metering system for injection of a reducing agent into the exhaust gas flow of an internal combustion engine for selective catalytic reduction, the metering system being connected/connectable to a reducing agent tank from which the reducing agent is taken and delivered by means of a delivery pump via a delivery line and is introduced into the exhaust gas flow of the internal combustion engine via at least one nozzle, it is especially advantageous that a Venturi tube is integrated into the delivery line and has a branch which discharges into the nozzle, at the outlet of the Venturi tube there being an openable shutoff element which is closed during metering operation and is opened when metering is completed so that with the shutoff element opened the reducing agent which is located in the nozzle and the branch is intaken as a suction medium into the Venturi tube and is carried away via the outlet.

The terms reducing agent metering system and metering system are used synonymously within the scope of this specification. The term reducing agent solution or reducing agent encompasses any reducing agent which is suitable for selective catalytic reduction, preferably a urea solution according to DIN 70070 is used for this purpose. But the invention is not limited thereto.

In one preferred overall arrangement the reducing agent metering system has a tank which is filled with the reducing agent solution and from which the reducing agent solution is taken and delivered by means of a metering pump and is introduced into the exhaust gas flow of the internal combustion engine via at least one nozzle.

In metering operation, i.e. during the metering and injection of the reducing agent into the exhaust gas line, the shutoff element at the outlet of the Venturi tube is closed so that the reducing agent is delivered to the nozzle via the branch. After completion of metering the shutoff element at the outlet of the Venturi tube is opened so that the delivery flow of the reducing agent flows from the inlet into the Venturi tube straight to the outlet of the Venturi tube and due to the acceleration of the flow in the Venturi tube the static pressure drops, as a result of which the reducing agent which is located in the branch line to the nozzle is intaken by the so-called Venturi effect according to the action principle of the jet pump and is carried away with the main flow via the outlet of the Venturi tube. In this way the delivery line and the nozzle can be completely evacuated. The outlet of the Venturi tube can be connected to the tank so that the amount which has been sucked back out of the branch line can be delivered back to the tank.

This reliably prevents freezing damage and also the formation of clogs by crystal formation. By placing the Venturi tube between the metering pump and the outlet opening of the nozzle and the switching of the shutoff element at the outlet of the Venturi tube, the delivery flow of the reducing agent during metering operation is delivered exclusively via the branch line to the nozzle, and upon completion or interruption of metering operation after opening of the shutoff element which is provided at the outlet of the Venturi tube, by the operation of the Venturi tube as a jet pump the delivery flow is intaken and carried away out of the nozzle and the branch line back into the Venturi tube. In this way the nozzle together with the nozzle opening and the delivery line which is located upstream are evacuated after completion of the injection of the reducing agent which could otherwise freeze or crystallize out and clog the nozzle.

The core of the invention is thus the arrangement of a Venturi tube which can be shut off in the delivery line on the pressure side of the delivery pump, which tube is used during operation as a simple elbow to the branch line toward the nozzle and upon completion or interruption of metering operation is used as a jet pump for evacuating the branch line and the nozzle. The invention devises a simple system with which it is possible to withdraw the liquid in the nozzle and in the feed line to the nozzle after completion of metering. Here the principle of the suction jet pump is used, i.e. the negative pressure which arises in the Venturi tube by accelerating the delivery liquid provides for the intake of the liquid from the nozzle and the feed line to the nozzle.

The arrangement of a connection between a compressed air system and the reducing agent delivery line is thus no longer necessary, as a result of which the reliability of the reducing agent metering system is increased and the systems engineering cost is reduced.

Preferably the branch is located in the region of a narrowing of the Venturi tube. As a result of this narrowing of the Venturi tube, the flow in the Venturi tube is accelerated. Based on the acceleration of the flow and of the associated rise of the dynamic pressure the static pressure in the delivered liquid drops in the region of the narrowing. When the shutoff element which is provided at the outlet of the Venturi tube is opened the liquid which is located in the branch line and the nozzle can be intaken due to this lowered static pressure so that the branch line and the nozzle are evacuated.

In one especially preferred embodiment the branch is located half in or directly in front of a cross sectional widening of the Venturi tube. Alternatively the branch can be located spaced upstream in front of a cross sectional widening of the Venturi tube.

The flow can be returned again to those flow states prevailing upstream of the Venturi tube with respect to the pressure in the fluid and with respect to the flow velocity by this cross sectional widening downstream of the branch.

In one especially preferred embodiment, the branch is located in the region of a propellant nozzle of the delivery line, which nozzle discharges into the Venturi tube. In this case the Venturi tube can be made like a jet pump by a propellant nozzle of the delivery line discharging into a chamber, the branch to the nozzle discharging into the same chamber. Downstream of the branch there can then be a cross sectional narrowing followed by a diffusor as far as the outlet of the Venturi tube. This configuration then corresponds to the typical structure of a jet pump.

The branch to the nozzle can be located vertically on the Venturi tube or however can include an acute angle with the Venturi tube. The acute angle can be formed in such a way and the branch can be located on the Venturi tube in such a way that the flow direction from the inflow into the Venturi tube and the discharge out of the branch toward the nozzle is deflected by up to 135°, especially by up to 150°. If the branch is located vertically on the Venturi tube, the flow with the shutoff element closed is deflected at the outlet of the Venturi tube by 90° so that the entire unit in this case is used like a simple pipe elbow.

But it has been shown that the arrangement of the branch on the Venturi tube such that between the inflow region and the branch an acute angle is enclosed, such that the flow during metering operation is deflected by 135° or by up to 150°, is especially advantageous and the evacuation of the branch line and the nozzle can take place very promptly and effectively when the shutoff element provided at the outlet of the Venturi tube is opened.

The inside diameter of the Venturi tube in the region of the branch can be less than or equal to 5 mm, especially less than or equal to 4 mm, especially less than or equal to 3 mm, especially less than or equal to 2 mm, especially less than or equal to 1 mm, especially less than or equal to 0.8 mm.

Alternatively or cumulatively the inside diameter of the branch can likewise be less than or equal to 5 mm, especially less than or equal to 4 mm, especially less than or equal to 3 mm, especially less than or equal to 2 mm, especially less than or equal to 1 mm, especially less than or equal to 0.8 mm.

In one preferred embodiment of the reducing agent metering system there is a compressed air supply, the reducing agent being atomized inside or outside of the nozzle by means of compressed air. To atomize the reducing agent there can be a mixing chamber within which atomization of the reducing agent by means of compressed air takes place prior to introduction into the exhaust gas line. But in one preferred embodiment the nozzle is made as an externally mixing binary nozzle in which the reducing agent solution emerges from a first nozzle opening and compressed air emerges from a second nozzle opening, the two nozzle openings being aligned to one another such that the compressed air atomizes the reducing agent outside of the nozzle so that the nozzle is made as an externally mixing binary nozzle and aerosol formation occurs outside of the nozzle.

Preferably there is a compressed air supply which has a switching valve and/or a pressure control valve. This switching valve is used for control, i.e. turning the compressed air supply on and off.

Alternatively or cumulatively the compressed air supply can have a pressure control valve. In this way the compressed air can be set to a pressure level which is desired for atomization of the reducing agent by means of compressed air. The compressed air itself can be taken from an embarked compressed air system, for example of a truck, in whose exhaust gas line the metering system is located, without the system pressure which is prevailing in the compressed air system constituting a limitation since the pressure of the compressed air can be reduced to the desired pressure.

The metering pump can be especially a membrane pump or a piston pump or a centrifugal pump or an orbital pump or a geared pump.

A membrane pump which is used as a metering pump can be triggered for example with a frequency of up to 50 Hz or up to 100 Hz. The triggering signal of the membrane pump can be pulse width modulated.

This pulse width modulation of the triggering signal of the membrane pump can improve the metering accuracy and the noise behavior. In particular it has been shown that the metering accuracy can be distinctly improved by the triggering of a membrane pump with a pulse width modulated signal. The instantaneous delivery of the membrane pump can be matched to the respective operating state of the internal combustion engine and to the temperature and mass flow of the exhaust gas by pulse width modulation of the triggering signal of the membrane pump so that the metering of the reducing agent can take place exactly, tailored to demand.

Several exemplary embodiments of the invention are shown in the figures and are detailed below.

FIG. 1 shows a first embodiment of a Venturi tube which is integrated into the delivery line with the branch to the nozzle;

FIG. 2 shows a second embodiment of a Venturi tube which is integrated into the delivery line with the branch to the nozzle;

FIG. 3 shows a third embodiment of a Venturi tube which is integrated into the delivery line with the branch to the nozzle;

FIG. 4 shows a fourth embodiment of a Venturi tube which is integrated into the delivery line with the branch to the nozzle;

Identical parts, components and modules are identified in the figures with identical reference numbers.

FIGS. 1 to 4 each show an extract from the delivery line 1 of a reducing agent metering system for injection of a reducing agent into the exhaust gas flow of an internal combustion engine for selective catalytic reduction. The metering system is connected to a reducing agent tank from which the reducing agent is taken and delivered by means of a delivery pump which is not shown via the delivery line 1.

A Venturi tube 2 which in the exemplary embodiments shown in FIGS. 1 to 3 has a narrowing 5 and furthermore in the region of the narrowing 5 a branch 3 is integrated into the delivery line 1. The branch 3 widens to a line diameter corresponding to the delivery line 1 and discharges into the nozzle which is not shown and via which the reducing agent is injected into the exhaust gas line of the internal combustion engine.

As can be taken from FIGS. 1 to 3, downstream of the branch 3 a cross sectional widening 6 of the Venturi tube follows. At the outlet 4 of the Venturi tube there is a switchable shutoff element by means of which the outlet 4 of the Venturi tube can be closed off.

During metering operation of the metering system the shutoff element at the outlet 4 of the Venturi tube 2 is closed so that the reducing agent which has been delivered via the delivery line 1 is delivered to the nozzle via the branch 3 and is injected into the exhaust gas line of the internal combustion engine.

When metering is completed the shutoff element at the outlet 4 of the Venturi tube 2 is opened so that the reducing agent which has been delivered by the delivery line 1 discharges via the outlet 4 of the Venturi tube 2. As a result of the narrowing 5 within the Venturi tube 2 the delivered liquid is accelerated, i.e. the dynamic pressure rises dramatically. As a result of this, the static pressure drops dramatically since the total pressure is constant. As a result of the Venturi effect which arises within the Venturi tube 2, in this way the fluid which is located in the nozzle and in the branch line 3 toward the nozzle is intaken and entrained by the main flow of the narrowing 5 of the Venturi tube 2 and carried away via the outlet 4.

In the embodiment as shown in FIG. 1 the branch 3 in the region of the narrowing 5 is spaced apart from a cross sectional widening 6 of the Venturi tube 2, which widening follows downstream. Here the branch 3 is located vertically on the Venturi tube 2.

In the version shown in FIG. 2 the branch 3 is likewise located vertically on the Venturi tube 2 and is likewise located in the region of the narrowing 5 of the Venturi tube 2. In the exemplary embodiment according to FIG. 2 the branch 3 is however not spaced upstream in front of a cross sectional widening 6 of the Venturi tube 2, but is located half in the cross sectional widening 6 of the Venturi tube 2, as can be taken from FIG. 2.

In the embodiment of the invention shown in FIG. 3 the branch 3 is located diagonally on the Venturi tube 2 so that an acute angle α is included between the branch 3 and the Venturi tube 2. This means that with the shutoff element closed, at the outlet 4 of the Venturi tube 2 the flow in this case is deflected by 135° into the branch 3 toward the nozzle which is not shown. It has been shown that this arrangement with an acute angle α between the branch 3 and the Venturi tube 2 is efficient especially for evacuation of the branch line 3 and the nozzle when the blocking element at the outlet 4 of the Venturi tube 2 is opened when the metering is completed.

In the embodiment according to FIG. 3 the branch 3 in the region of the narrowing 5 of the Venturi tube 2 is located half in a cross sectional widening 6.

In the exemplary embodiments according to FIGS. 1 to 3 the narrowing 5 of the Venturi tube 2 has an inside diameter of 1 mm. The branch line can likewise have an inside diameter of 1 mm or even a smaller diameter than the narrowing 5 of the Venturi tube 2, the branch line 3 then widening in the direction to the nozzle to a line diameter corresponding to the delivery line 1. But it has been shown that the branch line 3 and the nozzle can be especially efficiently evacuated when the branch line 3 in the connection region to the Venturi tube 2 has a diameter which is somewhat smaller than the narrowing 5 of the Venturi tube 2.

FIG. 4 shows another embodiment of the invention. Here a Venturi tube 2 is integrated into the delivery line 1, the Venturi tube in this case having the classic structure of a jet pump. From the delivery line 1 a propellant nozzle discharges into the mixing chamber 7 which has a cross sectional narrowing and is followed downstream by the diffusor 8. The branch 3 toward the nozzle is located in the region of the mixing chamber. Downstream, the Venturi tube at the outlet 4 has a switchable shutoff element which is closed during metering operation so that the delivered fluid discharges via the branch line 3 toward the nozzle and is injected into the exhaust gas line of the internal combustion engine. When metering is completed the shutoff element at the outlet 4 of the Venturi tube 2 is opened so that the overall arrangement according to FIG. 4 works like a jet pump and the reducing agent which has been delivered via the delivery line 1 is used as the propellant medium and the fluid which is located in the branch line is intaken as a suction medium into the mixing chamber 7 and is carried away via the outlet 4.

The embodiment according to FIG. 4 has an extremely complex structure, but it has been shown in tests that this embodiment also forms the most effective version for prompt and reliable evacuation of the branch line 3 and the nozzle.

The invention thus makes available a reducing agent metering system for injection of a reducing agent into the exhaust gas flow of an internal combustion engine for selective catalytic reduction, in which by means of one especially advantageous configuration of the delivery line 1, by integration of a Venturi tube 2 and a change of the flow direction by switching the switchable shutoff element at the outlet 4 of the Venturi tube 2 by a change of the flow direction a Venturi effect is produced which is used according to the principle of a suction jet pump in order to evacuate the branch line 3 and the nozzle and to intake the reducing agent which is located in the branch line 3 into the Venturi tube 2 and to carry it away via the outlet 4. The reducing agent solution which is to be metered is used at the same time as a propellant medium of a jet pump.

It is especially advantageous here that an extra line with compressed air or the like is not necessary for evacuating the system. Furthermore the reducing agent which is located in the branch line 3 can be returned again to the tank without its being injected unused into the exhaust gas line. Altogether the construction is distinctly simpler and the entire system is distinctly simplified compared to the prior art. When the outlet 4 of the Venturi tube 2 is connected to the suction line of the pump, after evacuating the branch line 3 toward the nozzle and the sensors, the pump is also partially ventilated; this is used to protect the entire system against freezing. 

1. A reducing agent metering system for injection of a reducing agent into the exhaust gas flow of an internal combustion engine for selective catalytic reduction, the metering system being connected/connectable to a reducing agent tank from which the reducing agent is taken and delivered by means of a delivery pump via a delivery line (1) and is introduced into the exhaust gas flow of the internal combustion engine via at least one nozzle, characterized in that a Venturi tube (2) is integrated into the delivery line (1) and has a branch (3) which discharges into the nozzle, at the outlet (4) of the Venturi tube (2) there being an openable shutoff element so that with the shutoff element opened the reducing agent which is located in the nozzle and the branch (3) is intaken as a suction medium into the Venturi tube (2) and is carried away via the outlet (4).
 2. The metering system as claimed in claim 1, wherein the branch (3) is located in the region of a narrowing (5) of the Venturi tube (2).
 3. The metering system as claimed in claim 1, wherein the branch (3) is spaced apart from the Venturi tube (2) or is located directly in front of a cross sectional widening (6) of the Venturi tube (2).
 4. The metering system as claimed in claim 2, wherein the branch (3) is located in the region of a propellant nozzle of the delivery line (1), which nozzle discharges into the Venturi tube (2).
 5. The metering system as claimed in claim 2, wherein downstream of the branch (3) there is a cross sectional narrowing (7) followed by a diffusor (8).
 6. The metering system as claimed in claim 2, wherein the branch (3) is located vertically on the Venturi tube (2) or wherein the branch (3) and the Venturi tube (2) include an acute angle (α).
 7. The metering system as claimed in claim 2, wherein the inside diameter of the Venturi tube (2) in the region of the branch (3) and/or the inside diameter of the branch (3) on the Venturi tube (2) is less than or equal to 5 mm, especially less than or equal to 4 mm, especially less than or equal to 3 mm, especially less than or equal to 2 mm, especially less than or equal to 1 mm, especially less than or equal to 0.8 mm.
 8. The metering system as claimed in claim 2, wherein there is a compressed air supply and the reducing agent is atomized inside or outside of the nozzle by means of compressed air, especially wherein the compressed air supply has a switching valve and/or a pressure control valve.
 9. The metering system as claimed in claim 2, wherein the metering pump is a membrane pump or a piston pump or a centrifugal pump or an orbital pump or a geared pump.
 10. A method for operating a reducing agent metering system for injection of a reducing agent into the exhaust gas flow of an internal combustion engine for selective catalytic reduction, the metering system being connected/connectable to a reducing agent tank from which the reducing agent is taken and delivered by means of a delivery pump via a delivery line (1) and is introduced into the exhaust gas flow of the internal combustion engine via at least one nozzle, characterized in that a Venturi tube (2) is integrated into the delivery line (1) and has a branch (3) which discharges into the nozzle, at the outlet (4) of the Venturi tube (2) there being an openable shutoff element which is closed during metering operation and is opened when metering is completed so that with the shutoff element opened the reducing agent which is located in the nozzle and the branch (3) is intaken as a suction medium into the Venturi tube (2) and is carried away via the outlet (4).
 11. The metering system as claimed in claim 2, wherein the branch (3) is spaced apart from the Venturi tube (2) or is located directly in front of a cross sectional widening (6) of the Venturi tube (2).
 12. The metering system as claimed in claim 3, wherein the branch (3) is located in the region of a propellant nozzle of the delivery line (1), which nozzle discharges into the Venturi tube (2).
 13. The metering system as claimed in claim 3, wherein downstream of the branch (3) there is a cross sectional narrowing (7) followed by a diffusor (8).
 14. The metering system as claimed in claim 3, wherein the branch (3) is located vertically on the Venturi tube (2) or wherein the branch (3) and the Venturi tube (2) include an acute angle (α).
 15. The metering system as claimed in claim 3, wherein the inside diameter of the Venturi tube (2) in the region of the branch (3) and/or the inside diameter of the branch (3) on the Venturi tube (2) is less than or equal to 5 mm, especially less than or equal to 4 mm, especially less than or equal to 3 mm, especially less than or equal to 2 mm, especially less than or equal to 1 mm, especially less than or equal to 0.8 mm.
 16. The metering system as claimed in claim 3, wherein there is a compressed air supply and the reducing agent is atomized inside or outside of the nozzle by means of compressed air, especially wherein the compressed air supply has a switching valve and/or a pressure control valve.
 17. The metering system as claimed in claim 3, wherein the metering pump is a membrane pump or a piston pump or a centrifugal pump or an orbital pump or a geared pump.
 18. The metering system as claimed in claim 1, wherein the branch (3) is located in the region of a propellant nozzle of the delivery line (1), which nozzle discharges into the Venturi tube (2).
 19. The metering system as claimed in claim 2, wherein downstream of the branch (3) there is a cross sectional narrowing (7) followed by a diffusor (8).
 20. The metering system as claimed in claim 6, wherein the inside diameter of the Venturi tube (2) in the region of the branch (3) and/or the inside diameter of the branch (3) on the Venturi tube (2) is less than or equal to 5 mm, especially less than or equal to 4 mm, especially less than or equal to 3 mm, especially less than or equal to 2 mm, especially less than or equal to 1 mm, especially less than or equal to 0.8 mm. 